Process for making high yield, DC stable proton exchanged waveguide for active integrated optic devices

ABSTRACT

A process for greatly improving the DC drift problem associated with LiNbO 3  and LiTaO 3  substrates. The process immerses the substrates in a heat bath comprised of a strong acid and then subjects the substrates to cyclical annealing.

This invention relates to integrated and guided-wave optical devices, including intensity and phase modulators.

More particularly, the invention relates to annealed proton-exchanged integrated optic chip (IOC) devices which each utilize a LiNbO₃ or LiTaO₃ substrate.

In a further respect, the invention relates to a process which greatly improves the DC drift problem commonly associated with LiNbO₃ and LiTaO₃ IOC devices, and which also reduces the insertion loss and improves the fabrication yield of such devices.

A LiNbO₃ crystal or substrate has several properties that are useful in photonics. The most useful property is the electro-optic (EO) effect. When an electric field is applied to the LiNbO₃ crystal, the refractive index is changed according to the strength of the electric field. If an optical wave is traveling through the crystal, the electric field also modulates the phase of the optical signal. The ability of the electric field to modulate the phase of the optical signal is called the electro-optic (EO) effect. The EO effect is, however, limited. To take advantage of the EO effect, a compact waveguide structure is formed on the LiNbO₃ (lithium niobate) crystal.

A lithium niobate crystal is processed in a wafer form in a manner similar to that utilized to process silicon crystals or wafers. A waveguide is formed in the lithium niobate crystal simply by increasing the refractive index in selected areas of the crystal. The waveguide ordinarily is only a few microns wide and thus is compatible with the dimension of an optical fiber.

Integrated optic chips (IOCs) were developed in the 1980s. Thousands of IOCs have been installed in CATV transmission equipment, digital telecommunication network equipment, fiber-optic gyros (FOGs), optical switches, and optical sensors. Advantages of IOCs include low-optical loss, low-voltage drive, high-frequency bandwidth, and small size and weight. Various types of IOCs are illustrated in FIG. 1.

Two prior art methods are utilized to fabricate LiNbO₃ devices. Both methods utilize conventional photolithography and vacuum deposit techniques found in semiconductor processing.

The first prior art method for fabricating LiNbO₃ devices is the titanium indiffusion process. During this process a titanium metal strip is deposited on a LiNbO₃ substrate. The substrate and titanium strip are heated to 1050 degrees C to diffuse titanium into selected portions or areas of the substrate to form waveguide channels in the substrate. Titanium ions diffuse to interstitial sites in the substrate and function collectively as an impurity dopant. The diffusion of titanium ions into the substrate increases the refractive indices of the substrate. This process was developed in the early 1970s.

The second prior art method is the proton exchange process. During this process a masked LiNbO₃ substrate is immersed in a molten bath of pure benzoic acid at a temperature below the decomposition point of the acid. The acid causes lithium ions at or near the surface of unmasked areas of the LiNbO₃ substrate to be replaced by hydrogen ions from the acid bath. Replacement of Li⁺ ions with H⁺ ions increases the refractive index of the unmasked areas of the substrate to form a waveguide channel(s) in the substrate. The proton exchange process was developed in the early 1980s and improved in the late 1980s.

The proton exchange process has major drawbacks. First, the process ordinarily is carried out with molten benzoic acid. Benzoic acid is a solid crystallized substance at room temperature. Consequently, the benzoic acid must be heated to a temperature over 200 degrees C. in order to carry out the proton exchange process. At this temperature, the vapor pressure of the molten benzoic acid is high and the process can become unstable.

In addition to the processing problems noted above, treatment of a LiNbO₃ substrate with benzoic acid produces severe crystal structure damage in the substrate. The substrate must be annealed to reduce the damage. Problems caused by crystal damage include high propagation loss due to scattering, instabilities in the refractive index of the waveguide channels, and degradation of the electro-optic effect. Annealing the substrate minimizes these problems. However, after annealing residual crystal damage still causes serious problems. One of the most important of such problems is DC stability. When an annealed IOC substrate (produced using a benzoic acid bath) is utilized for the Mach-Zehnder Interferometer (MZI) or switches, the IOC substrate may need to be biased with an electric voltage to operate properly. For example, in the CATV modulator application, a MZI needs to be biased at the so-called “quadrature” (π/2) point to achieve maximum linearity. For a switch application, a bias voltage is needed to maintain the device in minimum or maximum light transmission.

After a LiNbO₃ substrate is processed in a molten benzoic acid bath, the residual crystal damage and mobile charge migration present in the substrate cause the preset bias point for the IOC to drift from the original set point of the π phase over a period of time (See FIG. 2). This drift occurs at a very low frequency (0.001 Hz to 0.0001 Hz) and is called DC drift. The period of time which passes before such drift occurs can be as short as a few minutes or as long as several hours. Once drift occurs, the MZI is biased at an incorrect point and the modulated signal includes severe non-linearity. The DC drift may eventually even move beyond the maximum supplied voltage from the power supply. If this occurs, the entire transmission system must be reset in order to operate. Such DC voltage drift can therefore seriously compromise the integrity of the MZI.

DC drift is believed to occur because of the mobile charge and ions that are produced during treatment of a LiNbO₃ substrate in a benzoic acid bath. When a bias voltage is applied to the substrate, the mobile charge and ions in the waveguide region slowly move toward opposing polarity electrodes and set up a counter electric field which cancels the applied E-filed from the electrode. This is illustrated in FIG. 3.

The severity or magnitude of DC bias drift can vary among substrates processed in a molten benzoic acid bath.

One solution to the DC drift problem is providing a mechanism for resetting the bias point with a refreshing voltage directed to the bias electrodes of the IOC. The refreshing voltage restores the IOC to its original setting. The utilization of a refreshing voltage, however, requires the use of a monitoring system to measure the drift and initiate the necessary resetting sequence. This makes the system design much more complicated and renders the IOC useless for many applications.

In the past, sulfuric acid has sometimes been utilized to treat LiNbO₃ substrates. The use of sulfuric acid has not been preferred because it resulted in high loss and lower yield of serviceable substrates.

Accordingly, it would be highly desirable to provide an improved IOC substrate and method for making the same which would eliminate or minimize DC drift in the IOC.

Therefore, it is a principal objective of the invention to provide an improved IOC and method for making the same.

A further objective of the invention is to provide an improved IOC in which DC drift is minimized or is substantially eliminated so that a modulator or switch can be produced which can maintain its bias for extended periods of time.

These and other further and more specific objectives and advantages of the invention will be apparent to those skilled in the art from the following detailed description thereof, taken in conjunction with the drawings, in which:

FIG. 1 illustrates various kinds of integrated optic chips;

FIG. 2 is a graph illustrating the drift with time of the DC bias point in a conventional modulator or switch;

FIG. 3 illustrates theoretical mobile charge/ion migration to produce the opposite field to an applied voltage;

FIG. 4 is a graph illustrating the stability of the DC bias point of an IOC modulator or switch produced in accordance with the method of the invention; and,

FIG. 5 is a graph illustrating a multiple-ramp, multiple-soak annealing process carried out in accordance witch the principles of the invention.

Briefly, in accordance with the invention, I have discovered an improved integrated optic (IO) device. The device includes a crystalline material substrate having at least one surface; and, an integrated optic circuit. The circuit includes an optical waveguide and at least one electrode disposed in juxtaposition on the surface to provide at least one integrated optic region thereon. The optical waveguide is formed in the surface by a two step proton exchange process comprising the steps of immersing the substrate for a period of time in a heated bath including at least 70% by weight of a strong acid; and, cyclically differentially annealing the substrate by heating and cooling the substrate through at least first and second annealing cycles. Each annealing cycle comprises first heating the substrate through an up-ramp to an elevated temperature above room temperature, and then cooling the substrate through a down-ramp to a low temperature which is at least seventy-five degrees C. lower than the elevated temperature.

In another embodiment of the invention, I have discovered an improved method of fabricating a proton exchanged integrated optic device. The improved method comprises the steps of preparing a crystalline material substrate with at least one surface; forming channels in the surface to produce a selected integrated optic device waveguide; immersing the substrate in a heated bath of a strong acid; and, cyclically differentially annealing the substrate by heating and cooling the substrate through at least first and second annealing cycles. Each annealing cycle comprises first heating the substrate through an up-ramp to an elevated temperature, and then cooling the substrate through a down-ramp to a low temperature which is at least seventy-five degrees C. lower than the elevated temperature.

According to the invention, IOC waveguides of the type shown in FIG. 1 are produced using the following procedure.

1. A surface of a LiNbO₃ (or LiTaO₃) substrate is cleaned to produce a particle-free surface. A masking layer is fabricated on the substrate surface with a metal such as aluminum, gold or platinum, or, with a dielectric such as silicon dioxide or silicon nitride. The layer is produced utilizing conventional photolithography and/or vacuum deposition processes. The processes can be either an etch-away or lift-off process.

2. An acid resistant, sealable container is filled with sulfuric acid. The strength of the sulfuric acid can vary but is presently preferably in the range of about 70% to 98% by weight sulfuric or another strong acid. By way of example, 85% by weight sulfuric acid includes 85% sulfuric acid and 15% by weight water; 98% by weight sulfuric acid includes 98% by weight sulfuric acid and 2% by weight water. The invention is especially useful when a crystalline substrate is immersed in a heated strong acid like sulfuric, nitric, phosphoric, or hydrochloric (assuming the chlorine does not pose a contamination problem) acid. Hydrofluoric acid, though a strong acid, is not utilized because it excessively contaminates the substrate. Even though, for example, the use of sulfuric acid is sometimes referenced in the prior art, the damage caused to a LiNbO₃ substrate by such a strong acid has made use of strong acids impractical. The invention is believed, in contrast to such prior art experience which suggests strong acids are impractical, to permit a substrate to be placed in a strong acid at a relatively low temperature.

The LiNbO₃ substrate is loaded into a holder and submersed in the sulfuric acid. The container is closed. The sulfuric acid is heated utilizing conventional heating apparatus and temperature monitoring and control circuitry.

3. When the temperature of the sulfuric acid reaches a predetermined exchange temperature in the range of from about 100 degrees C. to 180 degrees C., the substrate is permitted to soak in the sulfuric acid for a selected period of time, typically in the range of about 70 to 200 minutes, and is then removed from the sulfuric acid. While the substrate is in the sulfuric bath, proton exchange occurs at the surfaces of the substrate not covered by the masking material.

4. The substrate is cleaned and the masked layer is etched off the substrate.

5. The substrate is loaded into a furnace and is annealed for two to twenty hours using multiple soak, ramped steps of the type illustrated in FIG. 5. In FIG. 5, the numbers “0”, “2”, “4”, etc. on the horizontal axis of the graph represent time in hours. Oxygen flows into the furnace at a rate of about 1 to 10 liters, preferably about two to four liters, per minute, at a relative humidity of 70% or more. Oxygen is the only gas introduced into the furnace and produces a pure oxygen atmosphere in the furnace. If desired, however, an oxygen—nitrogen or 100% nitrogen gas atmosphere can be utilized in the furnace instead of the oxygen gas atmosphere. Even hydrogen or helium could be utilized in the furnace. Hydrogen and helium are not normally practical because of cost or because of the danger of explosion. Pure oxygen is the presently preferred gas atmosphere in the annealing furnace or oven. The inclusion of water vapor in the furnace atmosphere is important in the practice of the invention. The water vapor is believed critical in removing OH⁻ ions from the substrate during annealing.

In FIG. 5, the substrate is placed into the furnace at room temperature and, during the first up-ramp, the temperature in the furnace is increased to a “high temperature soak” of 400 degrees C. The rate at which the temperature increases (on an “up-ramp”) or decreases (on a “down-ramp”) in the furnace is preferably in the range of one to twenty degrees C., preferably two to five degrees C., per minute. The furnace temperature is maintained at a high temperature soak of 400 degrees C. for a short period of time, then the temperature is down-ramped to a low temperature soak at 100 degrees C. The temperature is maintained at a low temperature soak at 100 degrees C. for a few minutes, and is then up-ramped to a high temperature soak of 350 degrees C. After a few minutes at a high temperature soak of 350 degrees C., the temperature is down ramped to a low temperature soak at 125 degrees C.; is up-ramped to a high temperature soak at 325 degrees C.; is down ramped to a low temperature soak at 150 degrees C.; is up-ramped to a high temperature soak at 300 degrees C.; is down-ramped to a low temperature soak at 100 degrees C.; and is then cooled to room temperature.

The cyclical heating and cooling of the substrate in the annealing furnace is believed critical in the practice of the invention, and requires that the substrate be subjected at least twice to an annealing cycle. The annealing cycle consists of heating (i.e., up-ramping) the substrate to an elevated temperature and then cooling (i.e., down- ramping ) the substrate to a temperature lower than the elevated temperature achieved during the preceding up-ramp. The substrate is preferably subjected to this annealing cycle three or more times. By way of example, when a substrate is heated (i.e., is up-ramped) from room temperature to 350 degrees C., is cooled (i.e., is down-ramped) from 350 degrees C. to 100 degrees C., is heated (up-ramped) from 100 degrees C. to 350 degrees C., and is cooled (down-ramped) from 350 degrees C. to room temperature, then the substrate has been subjected to two annealing cycles.

The temperature differential between the high temperature and the low temperature during each up-ramp (or down-ramp) cycle is also believed critical in the practice of the invention. If, for example, a substrate is heated from a room temperature of 50 degrees C. to 350 degrees C. and is then cooled or down-ramped to 150 degrees C., then the temperature differential during the down-ramp is 350 degrees C. minus 150 degrees C. which equals a temperature differential of 200 degrees C. The temperature differential during the up-ramp is 350 degrees C. minus 50 degrees C. which equals a temperature differential of 300 degrees C. In the practice of the invention, each up-ramp or down-ramp temperature differential should be at least 75 degrees C., preferably at least 100 degrees C. These temperature differentials are believed important because heating the substrate expands the substrate while cooling the substrate contracts the substrate. Such expansion and contraction of the substrate is believed to facilitate the movement of mobile charges in the substrate so they can be removed from the substrate—much in the same manner that periodic stretching of clothing by a washing machine facilitates removal of dirt particles from clothing in the washing machine.

While the high temperature to which a substrate is heated at the conclusion of an up-ramp can vary as desired, ordinarily the substrate is heated to at least 250 degrees C. at the conclusion of each up-ramp. Heating a substrate at the conclusion or “top” of an up-ramp to a maximum high temperature of only about 150 degrees C. is not preferred because the annealing process does not appear to have any noticeable effect. Heating the substrate at the conclusion of an up ramp to a high temperature between 150 degrees and 250 degrees, while having some annealing effect, is not presently preferred because the annealing effect is too small and processing of the substrate consumes too much time. The low temperature reached at the conclusion of a down ramp during the cyclical annealing of a substrate can be room temperature or lower, but is typically usually at least 50 degrees C. at the conclusion of each down-ramp, except during the final down-ramp of the annealing process when the substrate is normally cooled to room temperature.

During the cyclical annealing of a substrate in the presently preferred practice of the invention, the substrate is typically up-ramped or heated from a beginning temperature to a high temperature of at least 250 degrees C. and is down-ramped or cooled from said high temperature to a low temperature no lower than about 50 degrees C. The time for which a substrate is maintained at a particular high temperature or low temperature can vary as desired.

An up-ramp is a line indicating how the temperature of a substrate changes while the substrate is heated from a beginning temperature to a selected elevated temperature. The up-ramp line ordinarily is straight, but need not be (i.e., the rate at which the temperature of the substrate changes need not be constant during the entire up-ramp line). A down-ramp is a line indicating how the temperature of a substrate changes while the substrate is cooled from a beginning elevated temperature to a lower temperature. The down-ramp line ordinarily is straight, but need not be.

It is possible during the practice of the invention to up-ramp or heat a substrate and down-ramp or cool a substrate through a complete single annealing cycle and then to let any desired amount of time pass before the substrate is subjected to a subsequent up-ramp—down-ramp annealing cycle. Practically speaking, however, it is presently preferred that each subsequent annealing cycle occurs promptly after the prior annealing cycle (if any) is completed.

In order to facilitate the expansion and contraction of the substrate to promote the movement of mobile charges in the substrate, the temperature of the substrate presently changes at a rate of one degree to twenty degrees, preferably two to five degrees, per minute during an up-ramp or down-ramp in the annealing furnace. If the temperature change during an up-ramp occurs too slowly or too quickly, then the ability of mobile charges to move in the substrate appears adversely affected.

6. After annealing is completed, an electrode pattern in fabricated on the surface of the substrate utilizing conventional photolithography and vacuum deposition techniques to form a metal electrode (s) layer. The resulting IOC substrate is polished, diced, fiber pigtailed packaged, and wire bonded. The finished IOC is tested with a bias voltage to evaluate its response and DC stability. A typical DC stability in a biased directional coupler switch produced in accordance with the invention is illustrated in FIG. 4. 

Having described my invention in such terms as to enable those skilled in the art to understand and practice it, and having identified the presently preferred embodiments thereof, I claim:
 1. An active integrated optic (IO) device, comprising (a) a crystalline material substrate having at least one surface; (b) an integrated optic circuit, including an optical waveguide and at least one electrode disposed in juxtaposition on said surface to provide at least one integrated optic region thereon; as characterized by: (c) said optical waveguide being formed in said surface by a two step proton exchange process comprising the steps of: (i) immersing said substrate for a period of time in a heated bath including at least 70% by weight of a strong acid, and (ii) cyclically differentially annealing said substrate by heating and cooling said substrate through at least a first annealing cycle comprising first heating said substrate through an up-ramp to a first elevated temperature above room temperature, and cooling said substrate from said elevated temperature through a down-ramp to a first low temperature which is at least seventy-five degrees C. lower than said elevated temperature, and a second annealing cycle subsequent to said first annealing cycle and comprising first heating said substrate through an up-ramp to a second elevated temperature above room temperature and above said low temperature, and cooling said substrate through a down-ramp to a second low temperature which is at least seventy-five degrees C. lower than said second elevated temperature.
 2. The integrated optic (IO) device of claim 1 wherein said elevated temperature during said first annealing cycle is different than said elevated temperature during said second annealing cycle.
 3. The integrated optic (IO) device of claim 1 wherein said low temperature during said first annealing cycle is different than said low temperature during said second annealing cycle.
 4. The integrated optic (IO) device of claim 1 wherein said elevated temperature during said first annealing cycle is identical to said elevated temperature during said second annealing cycle.
 5. The integrated optic (IO) device of claim 1 wherein said low temperature during said first annealing cycle is identical to said low temperature during said second annealing cycle.
 6. The active integrated optic device of claim 1 wherein said first low temperature is greater than about 50 degrees C.
 7. The active integrated optic device of claim 1 wherein during each of said up-ramps and down-ramps the temperature changes at a rate in the range of one degree to twenty degrees per minute.
 8. The optic (IO) device of claim 1 wherein in (c)(ii) said substrate is cyclically annealed in an atmosphere containing water vapor.
 9. The optic (IO) device of claim 1 wherein in (c)(ii) said substrate is cyclically annealed in an atmosphere containing oxygen.
 10. A method of fabricating a proton exchanged integrated optic device, comprising the steps of (a) preparing a crystalline material substrate with at least one surface; (b) forming at least one channel in said surface to produce a selected integrated optic device waveguide; (c) immersing said substrate in a heated bath of strong acid; and, (d) cyclically differentially annealing said substrate by heating and cooling said substrate through at least a first annealing cycle comprising first heating said substrate through an up-ramp to a first elevated temperature, and cooling said substrate from said elevated temperature through a down-ramp to a first low temperature which is at least seventy-five degrees C. lower than said elevated temperature, and a second annealing cycle subsequent to said first annealing cycle and comprising first heating said substrate through an up-ramp to a second elevated temperature above room temperature and above said low temperature, and cooling said substrate through a down-ramp to a second low temperature which is at least seventy-five degrees C. lower than said second elevated temperature.
 11. The method of claim 10 wherein said first low temperature is greater than about 50 degrees C.
 12. The method of claim 10 wherein during each of said up-ramps and down-ramps the temperature changes at a rate in the range of one degree to twenty degrees per minute.
 13. The method of claim 10 wherein in step (d) said substrate is cyclically annealed in an atmosphere containing water vapor.
 14. The method of claim 10 wherein in step (d) said substrate is cyclically annealed in an atmosphere containing oxygen.
 15. A method of fabricating a proton exchanged integrated optic device, comprising the steps of (a) preparing a crystalline material substrate with at least one surface; (b) forming at least one channel in said surface to produce a selected integrated optic device waveguide; (c) immersing said substrate in a heated bath of strong acid; and, (d) cyclically differentially annealing said substrate by heating and cooling said substrate through at least a first annealing cycle comprising first heating said substrate through an up-ramp to a first elevated temperature, and cooling said substrate from said elevated temperature through a down-ramp to a first low temperature which is at least seventy-five degrees C. lower than said elevated temperature, a second annealing cycle subsequent to said first annealing cycle and comprising first heating said substrate through an up-ramp to a second elevated temperature above said low temperature, and cooling said substrate through a down-ramp to a second low temperature which is at least seventy-five degrees C. lower than said second elevated temperature, and a third annealing cycle subsequent to said second annealing cycle and comprising first heating said substrate through an up-ramp to a third elevated temperature above said second low temperature, and cooling said substrate through a down-ramp to a third low temperature which is at least seventy-five degrees C. lower than said third elevated temperature.
 16. The method of claim 15 wherein each of said first and second low temperatures are greater than about 50 degrees C.
 17. The method of claim 15 wherein during each of said up-ramps and down-ramps the temperature changes at a rate in the range of one degree to twenty degrees per minute. 